Energy from the sun reaches the earth as electromagnetic radiation which can be absorbed by suitable surfaces and converted to heat. In general, the use of various solar energy collection systems has been known in the art. For example, it has heretofore been known to employ air source heat pumps to make use of solar energy stored in the atmosphere. Additionally, it has been known to use solar water heaters for this purpose in which water is circulated through a solar collector to be heated and the heated water is used to transfer heat energy to the place desired. However, the storage material is not exposed to the sun's rays, but instead the water is circulated by pumps or thermosiphons through a solar collector and then through a heat storage device in a serial manner. In some cases, a large container of water itself may serve as the heat storage device. Diurnal variations in insulation and atmospheric heat makes these active solar energy collection systems far from ideal. Also, there are complexities in the pumps or circulating systems which make such systems expensive. Further, the solar collectors are positioned in expose locations, for example on a roof, where they are subject to battering by storms and where they quickly lose their heat into the atmosphere when the sun's rays are blocked by clouds in the winter.
Among the materials heretofore used as sensible heat storage materials, water has most frequently been employed due to its high heat capacity and ready availability and low cost. Stored heat is moved to an area to be heated by circulating the heated water or heat exchanging it with circulating air. Water has also been employed in the roof pond and storage wall concepts of solar heating. Also, rocks and stones placed in an insulated storage space and adapted to heating and cooling by passing air through the interstitial spaces within the assemblage of rocks or stones and circulating the air to the space to be heated has also been employed. There has also been employed a hybrid of rocks and water where a tank is located in the center of a rock storage bin and air is circulated through the rocks which collect heat lost from the water tank.
Additionally, Trombe has developed a system whereby a relatively massive masonary wall is positioned behind panes of glass with an air space between the glass and wall. Openings at the top and bottom of the wall allow for gravity circulation of heated air into the interior space to be heated at the back side of the wall. This interior (or enclosed) space is also heated by thermal radiation from the rear (inside) surface of the solar heated Trombe wall, but the Trombe wall blocks illumination and view.
More recently, heat of fusion heat storage has received some attention as a means of storing solar energy. Most recently, paraffin waxes and fatty acid materials have been suggested for use as heat of fushion heat storage materials but have not found acceptance. Sodium sulfate decahydrate has received the most attention in this area, but problems of precipitation and immobilization or encapsulation of solid anhydrous sodium sulfate during the freeze-thaw cycles has caused degradation of the system and prevented its wide acceptance and use. Disodium hydrogen phosphate dodecahydrate has also received attention, but its thermal efficiency gradually decreases with increasing numbers of freeze-thaw cycles due to the heptahydrate formation which is the equilibrium solid phase at the melting temperature of the dodecahydrate.
Recently, a product containing calcium chloride hexahydrate has been merchandised by Kalwall Corporation, of Manchester, New Hampshire. This product comprises thermal storage pods which can be assembled into a translucent thermal storage wall. Because of the very high expansion coefficient and anisotropic freezing and thawing characteristics of calcium chloride hexahydrate amount in effect to a volumetric change of approximately twenty percent during freezing/melting of this particular hydrated salt, a strong, flexible, expandable volume fiberglass-reinforced container must be used. Because of the fibrous nature of the necessarily reinforced container, a clear, optically transparent viewing through a wall of the Kalwall storage pods is unattainable.
Calcium chloride hexahydrate and any of the other water-and-salt eutectic thermal storage hydrates are unusable in planar, parallel sided, optically-transparent, rigid containers in a window configuration, for example, such as a window formed from two relatively closely spaced sheets of optically clear glass, because of their very high effective volumetric change during thawing/freezing. The anisotropic freezing/thawing characteristics of these water-and-salt hydrate eutetic materials, i.e. their tendency to form crystals during freezing which grow more rapidly in one direction than in other directions, augments the problems of attempting to employ them in a planar, parallel-sided optically transparent rigid container in a window structure.
A paper presented during the Fifth National Passive Solar Conference in Amherst, Mass. on Oct. 22 to 26, 1980, by J. R. Hull, J. F. McClelland, L. Hodges, J. L. Huang, R. Fuchs and D. A. Block, entitled Effect of Design Parameter Change on the Thermal Performance of a Transwall Passive Solar Heating System discusses a visually transparent thermal storage wall containing water and which is placed in building areas receiving direct solar radiation. A severe difficulty with such a water-filled external wall is that a power failure or heating system failure within the building during winter can allow rapid freezing of the water with resultant destruction of the entire wall due to the enormous expansion pressure of freezing water. Furthermore, thermal storage in water involves only a single-phase storage action in which the temperature of the water increases when heat energy is added and its temperature decreases as heat energy is withdrawn. Thus, the temperature of the associated enclosed space may be allowed to swing up and down over an uncomfortably large temperature range for the occupants during a twenty-four hour cycle. A further severe problem with such a thermal storage wall containing water is that a thickness of at least 5 cm (2 inches) is necessary to obtain at least 80% of the maximum solar savings fraction (beneficial results as defined in the article). Therefore, the weight when calculated on the basis of an entire building structure becomes adversely significant. Such a water-filled wall containing two inches of water weighs 10.4 pounds per square foot, for the water alone.
Such a single-phase acting thermal storage with its inherent temperature fluctuation is quite different from a phase change material in which the temperature remains approximately constant during addition or withdrawal of heat energy. The addition of heat energy causes melting of the material (change of phase from solid to liquid), and the withdrawal of heat energy causes freezing (change of state from liquid to solid), but the melting and resolidification occur at approximately constant temperature, called the "melting point". Therefore, among the advantages of a phase change material are those resulting from the fact that it tends to hold the temperature of the associated enclosed space more nearly constant than a single-phase acting material of equal thermal energy thermal energy storage capacity. In addition, a phase change material can store many times more thermal energy per unit volume than a single-phase acting material such as a body of water, a collection of rocks, or a Trombe wall. Thus, a considerably less bulky storage apparatus of phase change material has the same thermal storage capacity as a more bulky apparatus containing a much larger quantity of single-phase acting storage material.
There is, therefore, still a need for an improved solar energy heating unit that eliminates many of the undesirable features of systems heretofore proposed. A desirable system would be a solar energy heating system that employs many of the desirable features of heretofore studied systems but that eliminates and/or avoids most or substantially all of the undesirble features or drawbacks of prior systems. Additionally, it is highly desirable to have a system that employs a highly effective heat-of-fusion heat storage material that has a relatively high heat of fusion at the desired temperature range for heating a living or working space, yet which material is stable and does not separate nor segregate in use and is also non-toxic and non-corrosive. It is also desirable to have such a material that has a very small coefficient of thermal expansion. An acceptable material must be a phase change material that has a relatively high storage of thermal energy per unit of volume during its phase change.